1. Field of Invention
The invention relates to an optical scanning system for use with an image forming apparatus.
2. Description of Related Art
Conventionally, in an optical scanning system used in an image forming apparatus, such as a laser printer, image recording is performed by emitting a collimated beam of light from a light source, focusing the beam on a reflective surface of a deflector through a cylindrical lens placed in front of the deflector in a sub-scanning direction, and focusing the deflected beam on a surface to be scanned using an fθ optical system that scans with constant velocity thereon.
In a conventional fθ optical system, focal points in the main scanning direction and the sub-scanning direction are likely to vary back and forth in accordance with a scanning position. Therefore, in designing an fθ lens, attention has been directed toward reducing curvature of field to within an acceptable limit. When a laser beam with an inclination in the sub-scanning direction enters such an fθ lens, a deviation in the sub-scanning direction from an ideal scanning line varies according to a scanning position. This is caused because a focal length in the sub-scanning direction and the primary plane vary according to a scanning direction. As a result, magnification in the sub-scanning direction varies.
In addition, such a conventional fθ optical system needs a great power in a main scanning direction in order to launch a collimated beam in the main scanning direction, thereby the lens is likely to become large and thick. In particular, the manufacture of thick lenses in a plastic injection molding machine requires a long molding time, leading to cost increases. In addition, such a thick lens has a great difference in thickness between a center and an edge thereof, which makes molding difficult and stable performance can not be obtained.
To solve these problems, fθ scanning lenses which launch a convergent beam in the main scanning direction have been developed recently. However, because of complicated surface structure using a free curved surface, advanced technology is required for manufacturing such lenses, leading to cost increases.
For high printing speed and fine resolution, a multi-beam scanning optical system, which scans a plurality of lines at one time, using a plurality of semiconductor lasers or a semiconductor laser with a plurality of luminous points, has been developed. In the multi-beam scanning optical system, when magnification in the sub-scanning direction is different between a center, a start, and an end of a scanning line, a scanning line pitch varies in accordance with the magnification. If the scanning line pitch varies, the quality of image scanned degrades.